There are no direct links between Telligent, who provides CommunityServer and DotNetNuke, an Open Source initiative (directed by Shaun Walker from Perpetual Motions).

Let me point out the main differences

• DotNetNuke is free for all and open source (although there are commercial 3rd party modules and skin)
• DotNetNuke is a modular application framework, that can be configured and combined very flexible
• you can write DotNetNuke modules in either VB, C# or any other .Net language
• there are hundreds of free and commercial DotNetNuke modules available
• DotNetNuke framework comes with some basis modules, there are others that are developed either as core projects or individual from 3rd party developers. Until now DotNetNuke projects have contributed Blog, Repository, Forums, Gallery and Store (still beta) as well as enhanced UserDefinedTable. Many other projects are working on enhancement or rewrites of current core  and additional modules as Wiki, ...
• Have a look at these forums - they are created using the forum project's module. Do you really need more?

Some input on Business Directory / Classifieds on DNN:

The DNN Core Repository has a "Business Card" skin you can use when creating a business directory. I have used it for such a directory of businesses in a town in south america here:

http://www.santacruzperu.com/Contacts/HuarazDirectory/tabid/62/Default.aspx

The Repository module is very flexible with templating, customizable user upload/updates, ratings, comments etc. it also has a companion dashboard module with various designs (to summarize / show lastest, I use one on the homepage). I also use this module on the DotNetnuke websites directory that can be found here:

http://www.faktnet.com/DotNetNuke/DotNetNukeWebsitesDirectory/tabid/140/Default.aspx

The Core Repository Project has regular updates and the teamleader Steve Fabian & co are doing a great job with this module ( a new version should be out in a week or two). Here is the Project homepage:

http://www.dotnetnuke.com/Default.aspx?tabid=830

Other Ideas / options:

Biforce has a free Google Maps module with a directory listings feature, can be seen here:

http://dnn.bi4ce.com/Modules/GoMap/GoMapDirectory/tabid/288/Default.aspx

DNNJungle has a module (commercial for sale at snowcovered) called InfoMap, it can be used to created hierarchical InfoMaps showing ie. the businesses on a map with different icons for different types of business. This module is now using a provider interface, making it easy to write your own dataprovider. The module can be used for a lot of other use cases also some are demonstrated at the developers website:

http://dnnjungle.vmasanas.net/MyModules/Modules/InfoMap/tabid/35/Default.aspx

If you need hierarchical categories there is a developer called DotNetJedi that has commercial modules for Business Directory and Classifieds Directory. I have not used these modules, they can be found here:

http://www.dotnetjedi.com/DesktopDefault.aspx

 There are certainly other options (besides writing from scratch), welcome to the weird and wonderful world of DotNetNuke!